The basic structure using reduced surface field (RESURF) principle consists of a low-doped P-type substrate and a low-doped N-type epitaxial layer. A P well is formed on the epitaxial layer and N+, P+ are implanted into the P well, such that a transverse P-well/N-epi (P well/N-type epitaxial layer) junction and a longitudinal P-sub/N-epi (P-type substrate/N-type epitaxial layer) junction are formed. Due to a higher doping concentration at both ends of the transverse junction, the breakdown voltage of the transverse junction is lower than that of the longitudinal junction. The basic principle of RESURF is to enable the epitaxial layer to be completely depleted before the transverse junction reaches the critical avalanche breakdown field by using the interaction of the transverse junction and the longitudinal junction. By reasonably optimizing the device parameters, the breakdown of the device occurs in the longitudinal junction, thereby playing a role in reducing the surface field.
In an improved RESURF structure, a very low-doped N-type deep well is formed on the substrate or the epitaxy, such that a first RESURF is formed between the P-type substrate and the N-type deep well. Then, an implantation is performed in the N-type deep well and under the field oxide to form a P-type floating field limiting ring (FP), such that a second RESURF is formed between the FP and the deep well.
RESURF of such configuration satisfies certain requirements for high voltage, low on-resistance. However, the inventor found that in the tens of KHz to hundreds of KHz AC switch applications, the structure of the device may appear peak current, thus affecting the reliability of the device and the product, while the on-resistance cannot continue to be reduced.